Project Summary/Abstract The research plans and tailored training activities proposed in this Mentored Research Scientist Development Award (K01) application will enable the Candidate, Dr. Ashley N. Linden-Carmichael to develop essential skills to build her program of research and transition to independence as an alcohol investigator. Dr. Linden- Carmichael?s work uses innovative methodological and statistical techniques to gain an in-depth understanding of high-risk drinking patterns exhibited by young adults. The current application builds upon her previous work as a National Research Service Award (NRSA; F31) pre-doctoral fellow by studying an emerging public health concern: simultaneous alcohol and marijuana (SAM) use (i.e., using alcohol and marijuana at the same time). SAM use is particularly problematic because the use of both substances can interact to produce a synergistic effect on impairment. The proposed research activities aim to fill several gaps in our basic knowledge of SAM use by providing detailed, momentary information on the predictors and consequences of SAM use patterns through the innovative use of wearable alcohol sensors and ecological momentary assessments. Specifically, the two studies proposed aim to better understand SAM use patterns by (1) developing a brief measure of SAM use motives for use in ecological momentary assessment (EMA) surveys; (2) using EMA to identify person-, day-, and moment-level predictors of SAM use; (3) applying novel intensive longitudinal data methods to characterize dynamic features of alcohol intoxication within SAM and alcohol-only use episodes; and (4) comparing day-level consequences of episodes of SAM use versus use of either substance alone. This research will be conducted in The Edna Bennett Pierce Prevention Research Center at The Pennsylvania State University, an ideal and thriving research institution that will provide the Candidate with all of the resources she needs to carry out her research agenda and training goals. The Candidate will be mentored by an interdisciplinary team of experts with unique and overlapping skillsets: Dr. Stephanie Lanza (expert in intensive longitudinal data analysis), Dr. Jennifer Maggs (expert in theoretical frameworks for substance use across the lifespan), Dr. Joshua Smyth (expert in EMA methodology), and Dr. Emily Ansell (expert in momentary marijuana assessment). The career development plan outlined in this K01 application will allow the Candidate to obtain essential training on (1) problematic substance use behavior among young adults, (2) assessing high-risk drinking behavior and its correlates in the moment, (3) analyzing complex intensive longitudinal data, and (4) producing high-impact papers and successful grant applications to advance leadership in co-use. The research plan and training activities will facilitate Dr. Linden-Carmichael?s long-term career goal of building an independent program of research focused on understanding and intervening on problematic substance co-use patterns among young adult drinkers.